It is known that maleic acid in aqueous solutions does not readily form homopolymers, but will quite rapidly copolymerize with various other ethylenically unsaturated monomers, typically to form 1:1 molar ratio maleate:comonomer copolymers.
In most cases, maleate copolymers are prepared by a solution polymerization process employing organic solvents. For example, U.S. Pat. No. 4,710,537, issued Dec. 1, 1987 to Walinsky, teaches the preparation of a substantially homogeneous copolymer, consisting essentially of 35 to 65 mole percent acrylate and 65 to 35 mole percent maleate units and having a number average molecular weight of about 500 to 5,000. The copolymer is prepared in a chain-transfer solvent, preferably a ketone, ester, or ether having from 4 to 10 carbon atoms. Following polymerization, an aqueous solution of the copolymer may be prepared by adding water to the polymerization flask and distilling off the organic solvent.
For reasons of safety, efficiency in manufacturing and waste handling, it is desirable to polymerize the maleate copolymers in aqueous solution, rather than in organic solvent. An aqueous solution preparation of maleate copolymers is disclosed in Japanese Patent Application JP 60-212,410, published Oct. 24, 1985 by Fukumoto, et al. The polymerization method provides maleate copolymers with mean molecular weights of 300 to 5,000 and has a reported "polymerization rate" of, at most, 96.2%. The "polymerization rate" is the difference between the weight of monomer initially charged and the weight of copolymer recovered following polymerization (i.e., a measure of the unreacted residual monomer).
Another method for preparation of maleate copolymers in aqueous solution is disclosed in U.S. Pat. No. 4,519,920, issued May 28, 1985, to Fukumoto, et al. This process provides an initial, partial neutralization of the maleate monomer with alkali. A similar aqueous solution process for producing a polymaleate, with an option to copolymerize up to 20 mole percent of an unsaturated carboxylic acid comonomer, is disclosed in U.S. Pat. No. 4,668,735, issued May 26, 1987, to Fukumoto, et al.
U.S. Pat. No. 3,635,915, issued Jan. 18, 1972 to Gale discloses a process for preparing maleate/acrylate copolymers in aqueous solution using an inorganic peracid salt as a catalyst. The copolymers are prepared from 50 to 95 parts of acrylic acid and 5 to 50 parts of maleic acid, on a monomer weight basis (i.e., 57.5 to 96.5 mole percent acrylate and 3.5 to 42.5 mole percent maleate).
U.S. Pat. No. 4,659,793, issued Apr. 21, 1987, to Yang claims an aqueous polymerization process for manufacturing copolymer having a low residual free monomer content from carboxylic acid monomers and various comonomers. In the claimed process a metallic ion redox catalyst is used to prepare a copolymer comprising 25 to 55 weight percent (i.e., 21 to 47 mole percent) dicarboxylic acid monomer.
U.S. Pat. No. 4,314,044, issued Feb. 2, 1982 to Hughes, et al., claims an aqueous polymerization process for manufacturing copolymers of carboxylic acid monomers using a metallic ion redox catalyst system. As exemplified in the patent, the process only achieves about 95% monomer conversion. About 5 to 50 weight percent dicarboxylic acid is polymerized in the claimed process.
The processes for preparing maleate polymers and copolymers known in the art disadvantageously result in unacceptable levels of free, residual monomer, in particular, free, residual maleate monomer in the product of the polymerization. Such free, residual monomer is undesirable in many end use applications, particularly applications relating to personal care products, detergents and cleaners as well as industrial end uses involving further chemical reactions in which the residual monomer may participate as an undesirable reactant.
We have now discovered that an aqueous solution polymerization, employing a combination of at least 50 mole percent, preferably at least 70 mole percent charge preneutralization of the maleate monomer, a redox initiator catalyst using metallic ion, a slow feed addition of at least one comonomer, preferably over a period of at least three hours, to the neutralized maleate monomer and a post-feed of a polymerization initiator provides a copolymer having a weight average molecular weight of about 1,000 to 4,000, comprising at least 50 mole percent maleate, and less than 1%, on a copolymer weight basis, of residual, free monomer. This process may be carried out under ambient atmospheric conditions. An inert atmosphere, such as nitrogen, is not required for the polymerization. The resulting copolymer is useful as an antiscalant, an anti-incrustation agent, a dispersant, a detergent adjunct, a sequestrant and a water hardness salt (e.g., calcium carbonate, magnesium carbonate, magnesium sulfate, magnesium hydroxide, calcium sulfate, calcium silicate, magnesium silicate and barium sulfate) crystal modifier.